Exposes all root causes to ExceptionHandler methods

Closes gh-28155

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.result.method.annotation;
 
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.function.Function;
@@ -213,21 +214,30 @@ private Mono<HandlerResult> handleException(Throwable exception, HandlerMethod h
 
 		InvocableHandlerMethod invocable = this.methodResolver.getExceptionHandlerMethod(exception, handlerMethod);
 		if (invocable != null) {
+			ArrayList<Throwable> exceptions = new ArrayList<>();
 			try {
 				if (logger.isDebugEnabled()) {
 					logger.debug(exchange.getLogPrefix() + "Using @ExceptionHandler " + invocable);
 				}
 				bindingContext.getModel().asMap().clear();
-				Throwable cause = exception.getCause();
-				if (cause != null) {
-					return invocable.invoke(exchange, bindingContext, exception, cause, handlerMethod);
-				}
-				else {
-					return invocable.invoke(exchange, bindingContext, exception, handlerMethod);
+
+				// Expose causes as provided arguments as well
+				Throwable exToExpose = exception;
+				while (exToExpose != null) {
+					exceptions.add(exToExpose);
+					Throwable cause = exToExpose.getCause();
+					exToExpose = (cause != exToExpose ? cause : null);
 				}
+				Object[] arguments = new Object[exceptions.size() + 1];
+				exceptions.toArray(arguments);  // efficient arraycopy call in ArrayList
+				arguments[arguments.length - 1] = handlerMethod;
+
+				return invocable.invoke(exchange, bindingContext, arguments);
 			}
 			catch (Throwable invocationEx) {
-				if (logger.isWarnEnabled()) {
+				// Any other than the original exception (or a cause) is unintended here,
+				// probably an accident (e.g. failed assertion or the like).
+				if (!exceptions.contains(invocationEx) && logger.isWarnEnabled()) {
 					logger.warn(exchange.getLogPrefix() + "Failure in @ExceptionHandler " + invocable, invocationEx);
 				}
 			}

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ControllerAdviceTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -82,9 +82,10 @@ public void resolveExceptionWithAssertionError() throws Exception {
 
 	@Test
 	public void resolveExceptionWithAssertionErrorAsRootCause() throws Exception {
-		AssertionError cause = new AssertionError("argh");
-		FatalBeanException exception = new FatalBeanException("wrapped", cause);
-		testException(exception, cause.toString());
+		AssertionError rootCause = new AssertionError("argh");
+		FatalBeanException cause = new FatalBeanException("wrapped", rootCause);
+		Exception exception = new Exception(cause);
+		testException(exception, rootCause.toString());
 	}
 
 	private void testException(Throwable exception, String expected) throws Exception {

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingExceptionHandlingIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -138,7 +138,7 @@ public Publisher<String> handleAndThrowExceptionWithCause() {
 
 		@GetMapping("/thrown-exception-with-cause-to-handle")
 		public Publisher<String> handleAndThrowExceptionWithCauseToHandle() {
-			throw new RuntimeException("State", new IOException("IO"));
+			throw new RuntimeException("State1", new RuntimeException("State2", new IOException("IO")));
 		}
 
 		@GetMapping(path = "/mono-error")